Endoplasmic reticulum

The endoplasmic reticulum is the site of phospholipid synthesis, it can be described as a membranous labyrinth and makes up 60% of all the membrane of the cell

It has an outer and inner leaflet; phospholipid synthesis occurs on the outer leaflet, but the enzyme scramblase folds the inner leaflet so that the materials for synthesis are in the correct place.

Rough endoplasmic reticulum

The majority of ER in cells is rough, its rough appearence is due to ribosomes which stud its surface; many proteins must be synthesised dirctly into the ER (for example proteins destined for the lysosomes and secretory proteins)

The ribosomes that cover the rough ER are not structurally different from the ribosomes in the cytoplasm; in fact, they only become ER ribosomes when a signal peptidase is encountered:

• A protein is being transcribed in the cytoplasm, a signal peptidase is transcribed (signal peptidases generally lie close to the 5' end of mRNA)
• A signal recognition particle (SRP) recognises and binds to this signal peptidase and upon…